For next generation copper access, interest has been expressed in allowing Point-To-Multi-Point (P2MP) protocols. This has the claimed advantage that N users can be served from a single transceiver in the access node, with positive impact on cost, size, energy consumption and scalability of the access node.
However, copper loop plants generally consist of dedicated (or point-to-point) links from the access node to the respective subscriber premises. Physically, the copper twisted pair is not shared among multiple users. Applying P2MP to such network is therefore untraditional. This is very inefficient in terms of transmit power. Moreover, the desire for reverse power feeding from customer premises and for Plain Old Telephony Service (POTS) add further obstacles for efficient implementation of P2MP access networks.
In a traditional P2MP topology, the transmit signal is divided at every branching. Consider a star topology with N branches for connecting to N users. The transmit power P is divided into N equivalent signals with power P/N in each of the N branches. Without loss of generality we omit here the fact that in practice, the signal powers on different branches can differ if their impedances differ. Typically, Digital Subscriber Line (DSL) standards as well as other access or in-home standards define maximum transmit Power Spectral Densities (PSD) that can be put on a line. In a star topology, the PSD limitation will be enforced by the first segment that connects the transmitter to the N branches of the star. Thus each branch sees a PSD much lower than the imposed PSD that depends on the impedances of the different loops. Disregarding the different channel characteristics of the branches, the transmit power is allowed to be a factor of N larger than dictated by the PSD limitation, as the power will be split over the N subscriber loops. Thus the front end hardware needs to be able to transmit at very high powers, a factor of N higher than traditionally, which in turn imposes hard requirements on the performance and linearity of components such as the line driver. Also, it is incompatible with the power budget constraints due to the reverse power feeding requirement. The straightforward alternative is to base the front end on current design and accept the hit in received power of a factor of N. Typically, N is in the order of 8 to 24. This corresponds to a signal-to-Noise Ratio (SNR) reduction in the order of 9 to 14 dB. An example of such state of the art implementation is a Wilkinson power divider.